Cracking of isobutylene with steam to produce substituted acetylenes and diolefins



W W56 J. HAPPEL ET AL 2,763,770

CRACKING OF ISOBUTYLENE WITH STEAM TO PRODUCE SUBSTITUTED ACETYLENES ANDDIOLEFINS Filed Sept. 29, 1952 Moles (3 H formed/I00 moles (3 charged IO20 30 40 5O CONVERSION (moles C H decomposed/I00 moies C H charged)United States Patent fiice 2,763,703 Patented Sept. 18, 1956 CRACKING FISOBUTYLENE WITH STEAM TO PRODUCE SUBSTITUTED ACETYLENES AND DIOLEFINSJohn Happel, Yonkers, and Charles J. Marsel, New York, N. Y.

Application September 29, 1952, Serial No. 312,157

11 Claims. (Cl. 260-678) This invention relates to novel processes forthe production of acetylenic hydrocarbons, particularly methylacetylene, and certain diolefins, chiefly allene, by the thermalcracking of branched chain monoolefins, and more particularly it relatesto a new and superior method for making methyl acetylene by the thermalcracking of isobutylene using a combination of critical conditions.

It has been known to pyrolyze olefins to give diolefins and otherproducts. The pyrolysis of isobutylene has also been known to the art.Previously, in order to obtain a feasible and operative process, it hasbeen necessary to conduct the thermal cracking of the branched chainolefins, such as isobutylene, under very special conditions ofcontrolled temperature and under vacuum in order to obtain appreciableamounts of the more valuable products such as methyl acetylene. In fact,studies conducted on the chemical mechanism of the thermal decompositionof isobutylene during pyrolysis have indicated that this crackingoperation is extremely sensitive to operating conditions with regard toboth yield and identity of products as well as to quantity of worthlesstars and fixed gases obtained. Accordingly, the previously employedmethods used for obtaining acetylene from branched chain monoolefinsrequired the use of greatly reduced pressures as well as carefullycontrolled temperatures and contact times and was accompanied by theusual problems of operation at reduced pressures and high temperatures.These include expensive equipment, difliculties of control, poor heattransfer and uncertainty of products and yields. These difficulties, ofcourse, are intensified in commercial scale operations.

It has now been discovered that low molecular weight substitutedacetylenes and especially the valuable methyl acetylene, can be made ingood yield and purity by the thermal cracking of iso-monoolefins in thepresence of steam and under conditions of high temperatures and lowcontact time of the feed. By this new method, the diliiculties ofoperating under vacuum are completely avoided, since atmospheric or evenslightly superatmospheric pressures are quite satisfactory. There is nocoking of the equipment, and consequently no loss of olefin feed stockin such useless materials. There is obtained a product having maximumyields of both methyl acetylene and allene, both of which are highlyuseful products.

It has been discovered that steam has a unique and hitherto unknownability to aid the particular desired reaction by cutting down tarformation and greatly facilitating the formation of the desired mixtureof methyl acetylene and allene.

To summarize, it has been found that, by maintaining proper temperatureand contact time relationships, and controlling certain other secondaryvariables, maximum yields of methyl acetylene and allene from thermalcracking of isobutylene are obtained which are higher than thoseobtained by more inconvenient and less practical methods.

This relationship is shown graphically in the accompanying figure whichis a plot of the relation of total production of C3H hydrocarbons (i.e., allene and methyl acetylene), per unit of isobutylene throughputthrough a pyrolysis coil to the degree of decomposition of theisobutylene, expressed as a percentage, which takes place in the samepyrolysis coil. It will be noted that a maximum yield of CsH4hydrocarbons, the desired product, is reached at about 30% conversion.At conversions lower than the optimum, although the desired reactionoccurs to a considerable extent, the amount of unreacted isobutyleneremaining is undesirably high. At conversions higher than the optimum,however, a considerable portion of the C3H4 hydrocarbons produced aredecomposed by secondary reactions, resulting in a loss of the desiredproduct.

Therefore, these data and the graph clearly indicate that the pyrolysisreaction should be carried out at conversions in the range of about 20%to 40% based on the isobutylene used. For the temperature range of 800to 900 C., the corresponding contact times for 20-40% conversion havebeen determined. These data are shown below in Table I.

TABLE I Contact Time, Seconds Temp, 0.

20% Con- 30% Con- 40% Conversion version version Substantiallyatmospheric pressures are employed within the cracking zone for bestresults. Steam should be employed admixed with the reactive olefinicfeed in mole percent concentration of more than 50% and preferably inthe range of to mole percent. It is preferred to mix the iso-olefin andthe steam prior to its entrance into the reaction zone. A preheating ofthe steam is also desirable.

The feed can be the pure isobutylene or a mixture containing substantialamounts of this olefinic feed can also be used, particularly when it isinitially obtained as a commercial fraction as by distillation orextraction in petroleum operations or the like.

The operation may be carried out in a pyrolysis tube or a series oftubes or coils made of stainless steel, quartz or the like. Also, brickcheckerwork or stoves of the type used for pyrolysis may be used. Suchstoves commonly use the principle of regenerative cooling for economy ofoperation. Any such methods providing hot surfaces are quitesatisfactorily employed.

This novel process is particularly valuable for producing maximum yieldsof methyl acetylene from feeds of isobutylene or mixtures of olefinscontaining relatively large amounts of isobutylene. In addition, maximumyields of allene are obtained simultaneously with the methyl acetylene.Conditions which are especially adaptable for making methyl acetylenetogether with allene are SOD-900 C. and .O110 seconds contact time and8090 mole percent steam. Unconverted isobutylene may be recycled throughthe reaction zone to insure that all the isobutylene has reacted.

In order to achieve controlled low time of contact, it is necessary tocool the cracked gases very quickly to at least 500 C. after they leavethe thermal cracking zone. Shock or quick cooling of the cracked gasesserves at least two purposes. Firstly it serves to bring the temperatureof the reacted gases quickly below the pyrolytic temperature in order tokeep decomposition of the products and secondary reactions to a minimumand secondly,

it'reduc-es the temperature of the methyl acetylene as well as theallene to the lower temperatures at which polymerization reactions ofthese materials are at a minimum.

These two objectives may be accomplished by shock cooling of the crackedgases. A direct water quench may be. placed immediately after theheating Zone. The gases may be passed directly into a stream or spray ofcold water or oil. Cool gases may be mixed with the exit gasesimmediately after they leave the heating zone. The substituted acetyleneis separated by condensation of the steam, further compression to knockout additional "water, and subsequent pressure distillation to separatemethyl acetylene and allene from the reaction products. Recoveredisobutylene is desirably recycled back to the cracking coil.

The following example is a typical illustration of the process of theinvention but it is not intended that the invention be limited in anyway thereto.

Example A gas mixture containing mole percent isobutylene and 90 molepercent steam is passed through a stainless steel cracking tube. Athermocouple probe in the cracking tube gave a temperature profile overthe length of the tube of about 850-900 C., which included preliminaryheating of the feed prior to its entrance into the reaction zone. Thecontact time in the furnace was between 0.05 and 0.5 second. The productwas then quenched with additional steam. Upon analysis of the reactionmixture, it was found that a minimum of 5.6 mole percent of theisobutylene was converted to methyl acetylene in a single pass, andapproximately an equal amount of allene was simultaneously formed. The'remaining reaction product consisted of a mixture of isobutjylene,methane, hydrogen and small amounts of other hydrocarbons.

The production of methyl acetylene is of especial impenance, since thischemical compound has unique propcities which make it useful as acombustible fuel for welding operations and jet engines, and as achemical intermediate to yield new and valuable synthetic chemicalproducts.

What is claimed is:

1. A process for the production of methyl acetylene which comprisessubjecting a feed containing substantial amounts of isobutylene and morethan 50 mole percent of "steam to temperatures of from 800 to 900 C. atcontact times ranging from .01 to 10 seconds.

2. A process for the production of methyl acetylene and otherunsaturated products which comprises subjecting a hydrocarbon feedcontaining isobutylene and in the presence of more than 50 mole percentof steam, to temperatur'es of from 800 to 900 C. at a contact time ofless than 10 seconds.

3. A process for the production of CsHa unsaturated hydrocarbons whichcomprises subjecting a hydrocarbon feed containing isobutylene and inthe presence of more than SO mole percent of steam, to temperatures offrom 800 to 900 C. at a contact time of less than 10 seconds.

4. A process for the production of methyl acetylene whichcomprises-subjecting an olefinic hydrocarbon feed containing isobutyleneand more than 50 mole percent of 4 steam, to temperatures of from 800 to900 C. at a contact timeof less'than l second.

5. A process for the production of a mixture of methyl acetylene andallene which comprises subjecting a feed mixture containing isobutyleneand more than mole percent of steam to a thermal cracking at atemperature within the limits of 800 to 900 C. at a contact time of .01up to 10 seconds, and a conversion of isobutylene of from 20 to-40%.

6. A process which comprises subjecting a mixture of isobutylene andfrom to mole percent of steam, to a thermal cracking at a temperaturewithin'the limits of 800 to 900 C. at a contact time of less than 1second, and isolating methyl acetylene from the resultant cracked gases.

7. A process'which comprises subjecting a mixture containing from 10 to20 mole percent isobutylene and from 90 to 80 mole percent steam to acracking step at a temperature in the range of 800 to 900 C. at acontact time between .01 and 1 second and isolating methyl acetylenefrom the resultant cracked gases.

8. A process for making a mixture containing substantial quantities ofmethylacetylene and allene which comprises subjecting a mixture ofisobutylene and from '80 to 90 mole percent of steam to a thermalcracking at a temperature in the range of 800 to 900 C. and a contacttime of less than 1 second.

9. A process for making a cracked mixture containing methyl acetylenewhich comprises the steps of preheating a mixture containing about 10 to20 mole percent of isobutylene-and about 90 to 80-mole percent steam,immediately passing said preheated mixture to a thermal cracking at atemperature of 800 to 900 C. and contact time of -.O1 to 10 seconds,separating themethyl acetylene from the resulting c-rackedgases, andagain subjecting the unrea'cted isobutylene to the preheating andcracking steps.

10. Apr'ocess for-making a cracked mixture containing substantialamounts of methyl acetylene and allene which comprises'prehea'tingamixture containing 10 mole percent isobutylene and 90 mole percentsteam, passing said'mixtureto a cracking Zone in which it is subjectedto a temperature of' 850-900 C. for a period of about .05 to 0.5 second,quenching the hot mixture with steam, and isolating methyl acetylenefrom the cooled gases.

11. A process which comprises subjecting a mixture containing from '10to 20 mole percent isobutylene and from 90 to 80 mole percent steam to acracking step at a temperature in the range of 800 to 900 C., a contacttime between .01 and 10 seconds, and a conversion of isobutylene of from20 to 40%, and isolating methyl acetylene from the resultant crackedgases.

References Cited in the file of this patent "UNITED STATES PATENTS OTHERREFERENCES Groggins: Unit Proc'e'sses in Organic Synthetic, third ed.,page 704 (1947'), McGraw-Hill Book Co., Inc, New York.

1. A PROCESS FOR THE PRODUCTION OF METHYL ACETYLENE WHICH COMPRISESSUBJECTING A FEED CONTAINING SUBSTANTIAL AMOUNTS OF ISOBUTYLENE AND MORETHAN 50 MOLE PERCENT